11 research outputs found
PAMELA results on the cosmic-ray antiproton flux from 60 MeV to 180 GeV in kinetic energy
The satellite-borne experiment PAMELA has been used to make a new measurement
of the cosmic-ray antiproton flux and the antiproton-to-proton flux ratio which
extends previously published measurements down to 60 MeV and up to 180 GeV in
kinetic energy. During 850 days of data acquisition approximately 1500
antiprotons were observed. The measurements are consistent with purely
secondary production of antiprotons in the galaxy. More precise secondary
production models are required for a complete interpretation of the results.Comment: 11 pages, 3 figures, 1 table. Accepted for publication in Physical
Review Letter
Evidence of Energy and Charge Sign Dependence of the Recovery Time for the 2006 December Forbush Event Measured by the PAMELA Experiment
New results on the short-term galactic cosmic ray (GCR) intensity variation
(Forbush decrease) in December 2006 measured by the PAMELA instrument are
presented. Forbush decreases are sudden suppressions of the GCR intensities
which are associated with the passage of interplanetary transients such as
shocks and interplanetary coronal mass ejections (ICMEs). Most of the past
measurements of this phenomenon were carried out with ground-based detectors
such as neutron monitors or muon telescopes. These techniques allow only the
indirect detection of the overall GCR intensity over an integrated energy
range. For the first time, thanks to the unique features of the PAMELA magnetic
spectrometer, the Forbush decrease commencing on 2006 December 14, following a
CME at the Sun on 2006 December 13 was studied in a wide rigidity range (0.4 -
20 GV) and for different species of GCRs detected directly in space. The daily
averaged GCR proton intensity was used to investigate the rigidity dependence
of the amplitude and the recovery time of the Forbush decrease. Additionally,
for the first time, the temporal variations in the helium and electron
intensities during a Forbush decrease were studied. Interestingly, the temporal
evolutions of the helium and proton intensities during the Forbush decrease
were found in good agreement, while the low rigidity electrons (< 2 GV)
displayed a faster recovery. This difference in the electron recovery is
interpreted as a charge-sign dependence introduced by drift motions experienced
by the GCRs during their propagation through the heliosphere
Polyspermy prevention: facts and artifacts?
The purpose of this review is to open a debate as to whether or not oocytes actively repel supernumerary sperm or in nature final sperm : oocyte ratios are so low that polyspermy preventing mechanisms are not necessary. Before encountering the oocyte, spermatozoa need to be primed, either by environmental factors as in animals exhibiting external fertilization, or by factors from the female reproductive tract, as in mammals. The spermatozoon must then recognize and interact with the outer layers of the oocyte and progression of the fertilizing spermatozoon through these layers is further controlled and modulated by a precise sequence of signals in situ. Removal of these outer coats may not inhibit fertilization, however does interfere with the dynamics of sperm-oocyte interaction. We propose that monospermy in mammals and sea urchins, under natural conditions, is ensured by the controlled and gradual encounter of a minimum number of spermatozoa with the oocyte and that fine tuning is ensured by the structural and molecular organization of the oocyte and its surrounding coats. We suggest that laboratory experiments using oocytes deprived of their investments and exposed to unnaturally high concentrations of spermatozoa are artifactual and argue that the conclusions leading to the hypothesis of a fast electrical block to polyspermy are unfounded. Under laboratory conditions the majority of spermatozoa, although motile and capable of attaching to the oocyte surface, are either physiologically incompetent or attach to areas of the oocyte surface that do not support entry